To help determine the molecular basis of tooth eruption, this proposal will focus on how a requirement of eruption, osteoclastogenesis, is regulated at the molecular level. The dental follicle (DF), a loose connective tissue sac that surrounds the unerupted tooth, recruits mononuclear cells to the follicle where they fuse to form osteoclasts necessary to form an eruption pathway. A molecule produced by the DF, colony-stimulating factor-1 (CSF-1), is required for tooth eruption. Another molecule found in the DF, osteoprotegerin (OPG), inhibits osteoclastogenesis. In the first mandibular molar of the rat or mouse, the maximal number of osteoclasts is present either at day 3 (rat) or day 5 (mouse) on the alveolar bone, a time that correlates with maximal gene expression of CSF-1 in the adjacent follicle and minimal gene expression of OPG. It is postulated that the downregulation of OPG expression allows the maximal osteoclastogenesis and that this decrease in OPG expression is regulated by CSF-1. To test this hypothesis, osteopetrotic mice that have impaired CSF-1 production and inhibited tooth eruption will be examined to determine if OPG expression in the DF remains elevated at day 5, in contrast to its reduced expression in normal mice. Because injections of CSF-1 accelerate the time of eruption, it will be determined if such injections also reduce OPG gene expression. Conversely, injections with OPG will be done to determine if increased levels of OPG delay or inhibit eruption in normal mice and rats. Laser-capture microdissection techniques will be employed to resolve in what dental tissues of the rat receptor activator of nuclear factor-kappaB ligand is expressed to actively promote osteoclast formation. The final aim will determine what molecules may be involved in the secondary minor burst of osteoclastogenesis seen at day 10 postnatally in the 1st mandibular molar. Real-time PCR and microarray techniques will determine what genes are maximally expressed in the DF at this time. In vitro studies will determine if such gene products can support osteoclast formation. Understanding the molecular regulation of osteoclastogenesis during eruption may explain why impacted teeth (e.g., 3rd molars) do not erupt, as well as explain the causes of various eruption disorders. A molecular approach to induce eruption is a long-term goal. In addition, understanding the molecular basis of bone resorption may aid in determining what factors cause alveolar bone loss such as is seen in periodontitis.